Bean lock

Owned by Chris Hilder
Last updated: Dec 01, 2023
4 min read

The Kaffelogic is a transitional fluidisation roaster. This means that most roast profiles have been created to work on the threshold of fluidisation, that is, with a minimal amount of air. There is a delicate balance between the bean mass, density, and size, and the air flow required to circulate the beans.

This balance will change during the roast cycle.

If the airflow becomes too low at any time during the roast, the beans will stop circulating. This is called bean lock.

Bean lock might not happen with one origin, and start happening with a different origin. This is because density and size of the beans affects bean lock. The amount of chaff in the chaff collector also affects airflow: failure to empty the chaff collector between roasts may cause bean lock.

The different types of bean lock happen at different stages in the roast:

1. Initial bean lock

Beans not circulating properly at the start is known as initial bean lock. You can check circulation before starting a roast with Fan preview. If circulation is too low, correct it before starting the roast. It is recommended best practice to check circulation with fan preview whenever you use a different batch size or different beans.

If you start a roast with initial bean lock, the roast will usually complete without apparent problems, but the results will be uneven. In initial bean lock some beans will receive excess heat. This results in the affected beans cracking early. You might hear one or two beans crack loudly at a temperature of between 190°C/370°F and 200°C/400°F. The ‘early’ beans will be obvious in the final appearance of the roast as a dozen or so very black beans among the brown beans. See Uneven roasting.

Bean lock can occur initially and then reoccur later in the roast.

2. Early bean lock

Despite everything looking ok during fan preview, it is still possible for beans to lock later in the roast. As the beans swell during the roast they sometimes lock together and stop circulating. This is more common for load sizes of 150g or more and for darker roasts.

When the beans lock the temperature will usually increase steeply. This can cause wild upward fluctuations in the temperature line of the roast log.

When beans lock after starting a roast, but before 160°C/320°F, this is known as early bean lock. This will appear in the log something like this example:

3. Late bean lock aka “Thermal runaway”

When beans lock after 160°C/320°F this is known as late bean lock. The roaster is will normally detect late bean lock as thermal runaway, and stop the roast with the message: “Thermal runaway, press ⏵ to cool”.

Wild upward fluctuations in the temperature line will be apparent in the roast log prior to the roaster terminating the roast.

In rare cases thermal runaway will occur when bean lock has given rise to combustion of the bean mass in the roaster. In this case there will be billows of black smoke and, very rarely, flames. If this occurs do not initiate cooling, but immediately turn the roaster off at the wall, unplug it, and if safe to do so, take it outside or use your kitchen fire extinguisher. This is a rare situation, but as with any food cooking, combustion of the food is always a possibility. If this does happen to you, please follow up by contacting Customer Support for further guidance. In most cases after thermal runaway, cooling can be initiated and will complete normally. Then please follow the guidelines at Solutions, below.

4. Cooling bean lock

When beans lock only during cooling, this is known as cooling bean lock. This happens when the fan slows during cooling.

The beans will normally continue to cool normally despite not circulating. Circulation is not essential for effective cooling. Cooling bean lock is not necessarily a problem and you may choose to do nothing about it.

On the other hand, cooling bean lock can often be eliminated by increasing the ‘cooldown lo speed’ setting in Kaffelogic Studio.

Solutions

  1. Calibration: Ensure that your roaster fan speed is calibrated correctly. See Bean circulation calibration.

  2. Fan preview: Check circulation before starting a roast with Fan preview. If circulation is too low, correct it before starting the roast.

  3. Reduce quantity of beans: If fan speed is correctly calibrated and beans are circulating correctly at the start of the roast, but you are still experiencing early or late bean lock, the simplest and quickest solution is to reduce the batch size. For example, if you have loaded 120g into the roaster and experience bean lock, try loading 100g or 80g until you are roasting successfully. If you have BOOST do this without changing the entered load setting.

  4. Increase load setting: If you have BOOST, increase the entered batch size. For example, if you have loaded 120g into the roaster and experience early or late bean lock, try entering the load size as 140g or 160g. This will deliver more air to the same amount of beans.

  5. Custom profile: If you are creating your own profile you should consider editing the fan speed profile to deliver more air in the parts of the roast where beans are locking up. See Uneven roasting for more help on editing the fan speed profile. If you are dealing with cooling bean lock deliver more air in the late part of the cooling cycle by increasing the ‘cooldown lo speed’ setting in the profile.

Note

Avoiding bean lock can give rise to more beans in the chaff. This is dealt with using a technique described in Beans in the chaff.